1. Brief History of the Invention
During a cold evening in January 2003, the applicant discovered that the fireplace in his home was malfunctioning, emitting smoke into the room rather than expelling the smoke through the chimney. This has brought some concern to the applicant, and raised his curiosity as to the cause. Since that evening was unusually windy, wind was suspected as being the cause or the culprit. In the days and months following that incident, the applicant being a wind engineer who has taught wind engineering at University of Missouri-Columbia, and has written a book on the subject (see Reference 1), has been thinking about the incident, and trying to explain the phenomenon observed by using principles of fluid mechanics and wind engineering. Consequently, he has come to the realization that the reverse chimney flow that brought smoke into the room must have been induced by a low building internal pressure caused by wind. He also spent days in thinking to find a practical means to prevent such reverse chimney flow from occurring, and came up with the subject invention.
2. Operational Principles
This invention is based on the principles of building internal pressure, chimney effect, and stagnation tube, which are subjects of wind engineering, industrial aerodynamics, and fluid mechanics, respectively. Through a proper integration and utilization of these principles, which have not been done before in any prior art or practice, this invention was produced. In what follows, the three underlying principles are first reviewed briefly, and then their proper combination and utilization pertaining to the subject invention are described.
A. Building Internal Pressure
From wind engineering such as described in Reference 1, it is known that when wind blows over a building, it generates both an external pressure on the building exterior surface and an internal pressure inside the building. The external pressures on the windward wall and on the windward part of a steep roof are positive (i.e., above atmospheric or above ambient), whereas the external pressures on the side walls, leeward wall, and the roof (including flat roofs, roofs of mild slope, and the leeward part of any roof) are negative (i.e., below atmospheric or ambient). If the building has openings distributed approximately uniformly over various walls and the roof as it is usually the case, the internal pressure will be negative as explained in References 1 and 2. It is for this reason that the building internal pressure is usually negative during windy days, and the magnitude of this negative pressure (i.e., suction) increases as the square of the wind speed. This negative pressure inside buildings is detrimental to the proper operation of any type of indoor burners, which relies on the chimney to expel the combustion exhaust gases, including smoke, carbon dioxide and carbon monoxide, to the outdoor environment. The situation is especially dangerous when the combustion rate is reduced near the end of burning the last pile of wood in a fireplace at night when people were asleep. Due to the negative internal pressure or suction generated in the house or building by wind, the combustion exhaust including carbon monoxide and smoke can be sucked into the room rather than expelled outdoors through the chimney or flue. This can cause tragedy in winter time when one fails to extinguish the fire in a burner or fireplace before going to bed. Such tragedies can be prevented by controlling the internal pressure of the room in which the burner is used—making the internal pressure always positive.
This invention presents two effective and practical means to maintain a positive internal pressure needed for safe indoor combustion. The first means is to use an air pump, be it a fan, a blower or a compressor, to pump outdoor fresh air into the house, which will result in a rise of the building internal pressure. The second method is to use a specially designed stagnation tube, also called “simple pitot tube,” that is always facing the wind in order to generate a stagnation pressure at the tube inlet. The stagnation pressure in turn drives the outdoor fresh air into the pitot tube and through the connecting tubing into the room in which the burner is located.
B. Chimney Effect With and Without Wind
The “chimney effect” (also called the “stack effect”) refers to the rise of the hot buoyant gas exhaust through the chimney of a house or building—a phenomenon relied upon for proper operation of any chimney based on natural convection instead of forced (mechanical) convection. By using fluid mechanics as given in References 3, it can be proved that in the absence of wind, the flow of the exhaust gas through a chimney exists as long as the density (temperature) of the air entering the chimney is larger (smaller) than the density (temperature) of the exhaust gas leaving the chimney. However, in the presence of wind, the wind causes a negative pressure inside the building, counteracting the chimney effect in a manner analyzed and discussed in detail by Liu [4]. When the wind is sufficiently high, it generates a high negative building internal pressure, which in turn overpowers the normal chimney effect. In such a case, the exhaust gas from the burner, including carbon monoxide and smoke, cannot rise through the chimney. Rather, it flows back into the room in which the burner is located, causing a dangerous condition to the building occupants. Generally, the stronger the wind is, the greater this reverse chimney flow becomes and the greater the danger becomes. Therefore, proper control of the building internal pressure during windy days is the key to the prevention of the dangerous reverse chimney flow, and is the salient feature of this invention.
C. Stagnation Tube
To counteract the negative internal pressure generated by wind, which is the prime culprit for causing smoke and carbon monoxide to be sucked into the room during windy days, a stagnation tube is utilized in this invention to increase the building internal pressure. The concept of stagnation tube is explained in most fluid mechanics books and hence need not be explained here. Suffice to mention that it is based on the Bernoulli equation and the fact that when a tube with an open end is pointed into a flow of fluid (gas or liquid), the velocity of the fluid at the nose of the tube (i.e., the stagnation point) is always zero, and the pressure of the fluid there rises above the ambient pressure by an amount equal to 0.5 times (multiplies) the density of the fluid and times (multiplies) the square of the free-stream velocity. By attaching a vane to a pivoted stagnation tube, in a manner similar to attaching vanes to anemometers or some windmills, the stagnation tube opening will always be facing the wind. By connecting the other end of the stagnation tube to a pipe or tubing which in turn is connected to a room in which the building occupant is staying, the internal pressure of this room and other rooms is raised. This counteracts the detrimental effect caused by low internal pressure, and prevents the release of smoke and carbon monoxide into the building.
D. Integration and Utilization of the Three Principles to Yield the Subject Invention
The subject invention represents an integration and utilization of the three principles stated above. For effective use of these principles, the opening of the stagnation tube must be always pointed into the wind (i.e., facing the wind). This requires the use of a properly designed and properly constructed stagnation tube that can rotate in a horizontal plane, having a vane attached to the stagnation tube. Furthermore, the size of the stagnation tube and the size of the conduit (tubing or pipe) used to connect the stagnation tube to the room should be sufficiently large—say, at least of 0.25-inch inner diameter. Otherwise, the system will be ineffective because it will not significantly increase the building internal pressure.
3. Alternative Invention
An alternate embodiment of this invention is to use a small air pump (i.e., a fan or a blower) to draw fresh outdoor air into the house to increase the internal pressure whenever there is a build-up of carbon monoxide or smoke indoor caused by reverse chimney flow. In the event of any reverse chimney flow, it is much better to pump fresh air into the house, such as into one or more than one bedroom in which the occupants are sleeping, in order to build-up the internal air pressure and to restore the normal upward chimney flow through the burner, than to pump the indoor air out, which would cause a further reduction in the internal pressure, drawing more burner exhaust into the house.
The air pump of the alternate embodiment is to be driven by an electric motor, which in turn is controlled by a carbon monoxide or smoke sensor or detector. Whenever the sensor or detector measures a dangerous level of carbon monoxide or smoke indoor, the detector not only sends off an alarm but also triggers the motor, which turns on the air pump. The air pump sends the outdoor fresh air into the building and raises the building internal pressure. The increased internal pressure stops the release of chimney exhaust into the building, and sends the exhaust off the building through the chimney as it should be. This counteracts the dangerous reverse chimney flow generated by wind. In the event that the high level of carbon monoxide or smoke is generated by a blocked or dogged chimney, the fan bringing fresh air into the building is still very helpful because it causes venting of the building, which drives out the carbon monoxide and replaces it with fresh air. This shows that whatever may be the cause of the indoor buildup of carbon monoxide, having the air pump bring fresh air into the building and raise the building internal pressure is always effective in solving the problem. It is more effective than having the pump operate in reverse direction (i.e., expelling indoor air) as it is normally done in venting buildings, which decreases building internal pressure and draws more exhaust into the building through the chimney. This shows the merit of this invention.
4. Salient Features of the Invention
Salient features of the subject invention include the following:                The invention removes a major cause of smoke and carbon monoxide generated indoor—the reverse chimney flow generated by wind. This is in contrast to conventional carbon monoxide safety devices, which either trigger an alarm when the carbon monoxide concentration is higher than a threshold value, or triggers a ventilation system such as opening a window or turn on a fan to expel the room air containing high concentration of carbon monoxide to outside. As explained before, expelling the indoor air by a fan can be counterproductive because it further decreases the internal pressure of the room, drawing more smoke and carbon monoxide from the burner into the room. The subject invention removes this shortcoming.        Although the invention is intended mainly for preventing indoor release of carbon monoxide and smoke resulting from indoor burners, it is also effective to a lesser degree for reducing the danger caused by building fires. In the even of any building fire, the invented device (both embodiments) causes the building internal pressure to rise, which in turn causes a portion of the smoke in the building to enter and exit through chimneys. This reduces the amount of smoke spreading inside the building. Also, since the increased internal pressure directs the internal airflow towards chimneys which act as fire and smoke sinks, spreading of the fire indoor will be retarded by using the invented device. In spite of this benefit, one should never rely on this invention as the principal means for protection against building fires. Other conventional means, such as having fire sprinklers and smoke alarms, are more effective. Therefore, this invention should be used only as a secondary means for reducing the danger against building fires. Its principal purpose is for reducing the danger of carbon monoxide and smoke generated by indoor burners.        The invented stagnation tube system is entirely mechanical. It is wind powered, and it requires no battery or electrical power to operate. This is in contrast to all other conventional systems or devices to protect building occupants from the danger of carbon monoxide and smoke build-up, which requires either batteries or electricity supplied to the building. In the event of a drained battery, or an electrical outage happened to a home, all the conventional devices will fail, whereas the invented stagnation tube system will still be functional and will continue to offer protection when all the other devices will not.        Because wind generated reverse chimney flow is not the only cause for the generation of carbon monoxide and smoke indoors (for instance, a severely blocked chimney can cause the same to happen), this invented device should not be used as the sole mechanism to protect the safety of building dwellers from the danger of carbon monoxide exposure. It should be used in combination with other devices, such as a carbon monoxide alarm system, which is triggered whenever the level of carbon monoxide is high, whatever the cause of generation of the carbon monoxide may be. What this device does is it offers additional protection in the event other systems failed due to low battery or power outage, or other reasons.        Greatest protection against carbon monoxide and smoke can be achieved by using several systems simultaneously: (a) a carbon monoxide warning system and a smoke alarm powered by batteries, (b) the system invented here based on forcing air into the building using an air pump powered by the same ac electricity supplied to the building for lighting, and (c) the alternate embodiment invented here based on the stagnation tube, which does not require the use of electricity.        The invented stagnation tube system does not work without wind. However, when wind is not blowing, there is no negative internal pressure in a building, and no danger of the reverse chimney flow that forces the burner exhaust into the house. Thus, the device is not needed without wind. This shows that the invented method (device) works whenever there is indoor emission of burner exhaust gas caused by wind.        As with all other systems and devices, proper maintenance is needed to insure that the device invented here is functional all the time when an indoor burner is used. For instance, inspection of the stagnation probe mounted on the roof should be done at least once a year to make sure that the probe is not damaged by wind or other sources. More frequently, one should check the conduit (tubing) connecting the stagnation tube to the room, to make sure that it is not clogged. This can be done rather easily by one of several means, such as by connecting an inflated balloon to the outlet of the conduit. If the balloon is not deflated within a few seconds, the conduit must be clogged. Unclogging the conduit can be done easily by connecting a small compressor, such as those used in households for inflating bicycle tires or car tires, to the conduit outlet. The compressed air will purge the conduit clean of any dust or insects build-up.        
5. Description of and Comparison with Prior Art
Existing devices and methods for reducing the danger of having carbon monoxide in buildings are generally based on the detection (measurement) of the existence of carbon dioxide in a building, and then either to trigger an alarm (e.g., Reference 5), or to activate a venting mechanism, such as opening a window or starting a exhaust fan (e.g., Reference 6). Some more complex systems do both, i.e., triggering an alarm and activating a venting mechanism (e.g., Reference 7); they offer greater protection. However, all these systems depend on the use of electricity—either batteries or the alternating electrical current (60 hertz in the United States) supplied to buildings. Those that rely on batteries would not operate when the batteries are drained, and those that rely on the ac power would not work when the electricity is cut off, such as during a blackout. In contrast, the system invented here based on stagnation tube is wind powered; it needs no electricity to function. Through a literature search and patent search, the inventor has not found any existing device or method based on this mechanism.
The alternative embodiment of this invention, which is to pump outdoor air into the bedrooms of the house whenever there is a build-up of carbon monoxide in the house, is also unique. No prior art or invention based on the same concept has been found through a literature search and a patent search. The alternative embodiment is different from ordinary carbon monoxide venting systems which use fans to pump air out of the building, rather than into the building, which is less effective because they cause a reduction in the building internal pressure and a worsening of the revers chimney flow.